For a variety of reasons, it is frequently desired to mechanically suspend or isolate one system from another. For example, the control circuitry regulating a large, industrial punch press may be adversely affected by vibration and shock introduced by the press if the circuitry is directly secured to the press' frame. By suspending the control circuitry in a manner designed to maintain its orientation substantially independently of the punch press' operation, the life of the circuitry may be extended and its proper operation assured.
An even more common example of the need to suspend and isolate one system from another is found in the automobile. There, it is desirable to suspend the chassis of the vehicle from the axles upon which the wheels of the vehicle ride. This not only increases passenger comfort by producing a smoother ride, but significantly enhances the mechanical life of the body and engine by reducing the mechanical shock and fatigue each is subjected to.
The problems such systems must resolve are numerous. For example, in the context of the automobile suspension system, the system may be required to suspend a wide variety of loads. With only the driver riding in the vehicle, the load applied to the suspension system is relatively small. On the other hand, if the driver is also transporting passengers and cargo, a substantially greater load is applied to the suspension system. The system must be able to compensate for such variations in loading or its ability to suspend and isolate the chassis will be impaired. More particularly, without some form of compensation, the driver of an otherwise empty automobile would most likely experience a rough ride in which relatively little movement of the chassis with respect to the wheels occurs. With the vehicle fully loaded, on the other hand, the uncompensated suspension system would produce a relatively soft ride in which the chassis undergoes significant displacements with respect to the wheels.
The disruptive influence of variable forces applied to the suspended load presents another problem that must be addressed by many suspension systems. For example, with a punch press operating at different cycle rates, control electronics suspended from the press may be subject to different frequencies of vibration. Similarly, an automobile traverseing road discontinuities of varying separation may expose its chassis to a wide range of vibrational frequencies. The suspension systems employed in these applications should be able to compensate for the effect such variations have on the system to be suspended.
With respect to the automobile suspension system, road and driving conditions introduce additional forces with which the suspension system must contend. More particularly, during cornering of the vehicle, the chassis may be subject to forces normal to the direction of travel. These inertial cornering forces tend to alter the lateral orientation of the chassis with respect to the road surface, making it difficult for the passengers to comfortably maintain their seating. The suspension system should compensate for this roll or bank experienced by the chassis during cornering, thereby enhancing both passenger comfort and driver control of the vehicle.
Similarly, abrupt acceleration or deceleration of the vehicle may significantly affect the longitudinal orientation of the chassis with respect to the roadway. More particularly, during acceleration, the chassis may experience a condition known as squat, in which the forward portion of the chassis rises in relation to the rear portion of the chassis. During braking, on the other hand, a condition known as dive may occur, in which the front of the chassis drops with respect to the chassis rear.
To provide the desired isolation and suspension of one system with respect to another, a number of suspension systems have been designed having complexities that are somewhat proportional to the magnitude of the suspension problems presented. For example, in applications where the load is relatively insensitive to motion of the reference system from which it is suspended or where the load is subject to relatively few external forces, a simple vibration damper, such as a rubber pad, may be employed. The function of the damper is to absorb energy that would otherwise be transferred between the load and the reference system. While this manner of suspension has the obvious advantages of being relatively simple and inexpensive, its use, as noted above, it primiarly limited to applications where the reference system has relatively uniform operating characteristics and the suspended load requires relatively little isolation.
A slightly more sophisticated suspension system includes a spring added between the load and the reference systems. The spring stores energy that might otherwise be transferred between the load and the reference systems as the relative position of the two change. The stored energy may then either be released by a return of the load and reference systems to their original relative positions or be dissipated by the action of the damper. The precise manner in which this energy transfer occurs depends upon whether the response of the suspension system is overdamped, underdamped, or critically damped. In each instance, however, the suspension system reduces the energy transferred to the load, improving its isolation from the reference system. The spring constant of the spring, used to determine its operation in accordance the Hooke's law, may be either constant or nonuniform over the active operating length of the spring, depending upon the energy storage characteristics desired. This combined spring-and-damper arrangement provides slightly greater isolation of the load system from the reference system, particularly when more complex motion in the reference system is involved.
One common example of this elemental spring-damper suspension system is the standard automobile suspension system. Typically, automobile suspension systems including a spring and damper associated with each wheel. For example, the suspension associated with each front wheel may employ a relatively stiff cylindrical spring having a constant spring rate. The top of this spring is secured to a front cross member of the chassis and the bottom of the spring is attached to a lower control arm pivotally connected to the chassis. A shock absorber is generally located along the longitudinal axis of the spring and has its upper stem connected to the chassis and its lower stem fastened to the lower control arm. With this system employed, when the wheel traverses discontinuities or changes in the road surface, the force applied to the wheel develops potential energy in the spring rather than kinetic energy in the chassis, allowing the chassis to maintain its orientation with respect to the roadway. The damping effect of the shock absorber decreases the amount of potential energy originally stored in the spring and assists in its subsequent dissipation.
In rear wheel automobile suspension systems, a leaf spring having a nonuniform spring constant is often employed. The ends of the leaf spring are typically attached to the chassis and the center of the spring is secured to the rear axle housing. A rear shock absorber is also provided for each wheel, with the upper end of the shock absorber attached toward the center of the chassis to provide greater stability and the lower end attached to the rear axle housing.
While suspension systems that include both a spring and damper are typically more effective than systems employing only a damper, they may stil be inadequate when the load is subject to forces applied along more than one axis. For example, such a system may inadequately isolate the chassis of an automobile from movement of the wheels during cornering or sudden changes in acceleration or deceleration of the vehicle. To compensate for such phenomena, automobile suspension systems frequently include several additional components. For example, a stabilizer bar may connect the lower control arms to which the spring and shock absorbers of the left and right front wheel suspension systems are attached. This linkage of the two pivoting control arms reduces the tendency of the car to roll when cornering. Similarly, brake reactin rods may be provided, connecting the lower control arm of each front wheel suspension system to the automobile chassis. The function of the reaction rods is to maintain the position of the lower control arms with respect to the chassis, thereby resisting the tendency of the chassis to dive or squat when the automobile undergoes abrupt braking or acceleration.
While the stabilizer bar and reaction rods do reduce the affect of nonvertical forces applied to the chassis, they provide only limited relief. In addition, these elements do not provide the operator with control over the elevation of the chassis with respect to the wheels. The stablizer bar and reaction rods likewise do not offer the operator control over the relative compliance of the suspension system.
One arrangement intended to allow some control over the elevation of the chassis with respect to the wheels employs shock absorbers having a pressure tube that can be charged with varying volumes of air. With the vehicle subject to a particular passenger and cargo load, the elevation of the chassis with respect to the wheels is directly proportional to the volume of air provided to the shock absorbers. Such shock absorbers are commonly employed in rear suspension systems, which may undergo significant load variations. For example, when a vehicle is used to pull a relatively heavy trailer, the rear suspension system must support a substantial load that is not normally present. A rear suspension system unequipped to control chassis elevation would result in the rear of the chassis riding disproportionately low with respect to the forward portion of the chassis. By controlling the amount of air introduced into the shocks described above, however, a level ride can be produced under a wide variety of load conditions. While a suspension system employing such adjustable shock absorbers does provide the operator with some control over chassis elevation, the control typically is only exerted at specific occasions when the vehicle load is altered, rather than continuously while the vehicle is operating.
As an alternative to the foregoing arrangements, and a way of providing more continuous elevational control, air suspension systems have sometimes been employed. For example, in an automotive suspension system, such a system may include air spring units, elevational or leveling valves, a manual control valve, an air compressor, and an air storage tank or accumulator. Typically, an air spring unit includes an air chamber that is sealed on one end by a diaphragm and is fastened to the chassis of the car. A plunger is secured to the lower control arm and acts against the diaphragm, tending to collapse the diaphragm into the air chamber. Under a constant vehicle load, when additional air is pumped into the chamber the action of the diaphragm against the plunger causes the elevation of the chassis to increase. As air is removed from the chamber, the plunger collapses the diaphragm further into the chamber, decreasing the elevation of the chassis.
Automatic elevational control is provided by leveling valves secured to the front and rear of the chassis. These valves are mechanically actuated by elements that indicate the relative position of the chassis with respect to the wheels and that automatically trigger the leveling valves when variations in vehicle loading are experienced. Thus, as the locations of the position-indicating elements change in response to loading variations, the links between the elements and the leveling valves cause the valves to adjust the air supplied to the chambers until the original positions of the elements are restored. A manual control valve also allows the air supplied to the air spring units to be regulated and, with the control valve located inside the car, the driver can adjust the elevation of the chassis during operation of the vehicle. In either case, the pressurized air required for operation of the system is obtained from an accumulator tank charged by a belt-driven air compressor.
While the air suspension system described above does allow somewhat continuous elevational control of the chassis to be maintained, it suffers from several shortcomings. For example, the system provides relatively little control over compliance, has a relatively slow response time, and has a limited ability to accurately position the chassis with respect to the wheels.
To overcome the various problems involved in the suspension and isolation of one system from another, a suspension system should offer control over the elevation or displacement of the load system, as well as control over the compliance of the suspension. Elevation and compliance should be controllable in response to both manual inputs and automatically sensed operational changes. The response to such inputs and changes should also be both quick and accurate. Finally, the suspension system should be relatively lightweight and simple.